3.1.2 Additional particulars of the file format

In releases prior to 5.8 (2011-08-30), the grammar included the
production:

newphrase ::= id word* ";"

and used it in the admin, delta and deltatext
productions.
This allowed third-party programs to interoperate with RCS
by storing opaque (to RCS) data in the file.

As of 5.8, in the name of progress (towards more systematic file
integrity support), the only area reserved for third-party interop is in
the (string) value of the integrity field, specifically after the
first formfeed (U+0C).
A further restriction (for all programs) is that the integrity
value must not contain ‘@’.

Whitespace has no significance except in strings.
However, whitespace cannot appear within an id, num, or
sym, and an RCS file must end with newline (U+0A).
Strings are enclosed by ‘@’ (U+40).
If a string contains a ‘@’, it must be doubled; otherwise, strings
can contain arbitrary binary data.

Identifiers are case sensitive.
Keywords are in lower case only.
The sets of keywords and identifiers can overlap.

Dates, which appear after the date keyword, are of the form
Y.mm.dd.hh.mm.ss, where Y is the year, mm the month
(01–12), dd the day (01–31), hh the hour (00–23),
mm the minute (00–59), and ss the second (00–60).
Y contains just the last two digits of the year for years from
1900 through 1999, and all the digits of years thereafter.
Dates use the Gregorian calendar; times use UTC.

The delta nodes form a tree.
All nodes whose numbers consist of a single pair (e.g., 2.3, 2.1, 1.3)
are on the trunk, and are linked through the next field in order
of decreasing numbers.
The head field in the admin node points to the head of
that sequence (i.e., contains the highest pair).
The branch node in the admin node indicates the default
branch (or revision) for most RCS operations.
If empty, the default branch is the highest branch on the trunk.

All delta nodes whose numbers consist of 2n fields (n
≥ 2) (e.g., 3.1.1.1, 2.1.2.2) are linked as follows.
All nodes whose first 2n-1 number fields are identical are
linked through the next field in order of increasing numbers.
For each such sequence, the delta node whose number is identical
to the first 2n-2 number fields of the delta nodes
on that sequence is called the branchpoint.
The branches field of a node contains a list of the numbers of
the first nodes of all sequences for which it is a branchpoint.
This list is ordered in increasing numbers.
See Figure 3.1.